Based on the important factors of influencing the freshness of vegetables and the research results of the American ASHRAE on vegetable preservation conditions, it is well known that 90% of vegetables need to be in fresh-keeping conditions of a temperature ranging from 0° C. to 10° C. and a humidity ranging from 85% to 95%. Currently, the temperature of the fruit and vegetable refrigerating chamber of the existing refrigerators in the market satisfies the temperature requirement of keeping vegetables fresh. However, the humidity of the fruit and vegetable refrigerating chamber is about 50% and does not meet the humidity requirement, which severely affects the preservation quality of foods, such as vegetables, and causes a relatively short fresh-keeping period of the vegetables in the refrigerator because of losing moisture rapidly. Particularly, the green leafy vegetables will wither and won't be edible after seven days. Although some refrigerators are equipped with a fresh-keeping container to store vegetables for preventing the moisture loss, the fresh-keeping container only functions to slow down the moisture loss in practical, that is, cannot avoid the actual moisture loss of the green leafy vegetables and cannot perform the humidifying function. As a result, after the preservation period of seven days, the problems of withering and being not edible still exist, and thus the long-term preservation cannot be realized.